This paper presents an approach for the manufacturing tolerances optimization under the positioning effect.This work is organized around two axes. The first axis addresses the problem of manufacturing errors due to the free position of the part (without tightening). In this approach, we have modeled the positioning defects by using the small displacement torsor to determine the optimal distribution of the supports (plane support, linear support and point support). The second axis is dedicated to the modeling and positioning optimization under the tightening effect. In this section the defects have been modeled by the reaction torsors under the optimal position constraint. These models have been exploited to develop a tool for the optimal distribution, calculation of manufacturing dimensions, deviations and torsors of small displacements.